The picture is complicated by the frequent overlap of symptoms. Patients can be diagnosed with both disorders at the same time, (except when symptoms of anxiety occur exclusively within an episode of depression).

While depression and anxiety appear to stem from the same genes, they’re likely to arise from different environmental experiences. And the overall cluster of symptoms and potential treatments are very different.

Patients with generalised anxiety disorder tend to be hyper-vigilant to negativity and worry about the future. They believe they’re likely to encounter threats and are not able to cope with the anxiety this causes.

In contrast, patients with major depressive disorder tend to ruminate more on self-relevant negative information, focusing on what they see as true of themselves rather than on what might happen.

There’s also an increasing recognition of the role of dopamine – linked to reward and pleasure – in depression, but not anxiety.

A 2007 conference examined the evidence on the distinctiveness of depression and generalised anxiety disorder and concluded the two disorders were different. Another issue became clear: further research was needed to investigate their cause.

Does this classification really matter?

This isn’t just an academic debate. Distinguishing between mood and anxiety disorders and identifying the course they may take is important for a number of reasons.

Firstly, the risk of suicide is higher in individuals with anxiety or a combination of anxiety and depression, rather than depression alone.

Secondly, patients with anxiety have an increased risk of heart disease than those with depressive disorders. Our (unpublished) research shows this may stem from long-term adverse changes in the autonomic nervous system.

These changes are more prominent in patients with generalised anxiety disorder than other anxiety disorders – this may be due to a constant fear of not coping, coupled with a heightened perception of threat.

An accurate diagnosis and prognosis is important for determining the best course of treatment. Should the patient be recommended some form of psychotherapy? Or would an antidepressant medication or a combination of treatments be more appropriate? These are tough questions to answer.

Today, psychiatric disorders are diagnosed on the basis of certain features observed during a clinical interview. In the future, it’s possible that biomarkers could help determine the nature and course that a particular depression or anxiety will take.